1. Field of the Invention
The present invention relates to a photo diode array that can realize a large aperture ratio.
2. Background Art
One of semiconductor light receiving elements is a photo diode. The photo diode is an element for detecting light, wherein a reverse bias voltage is applied to a p-n junction, and the electron-hole pairs (photo carriers) generated by the incident of light are taken out as a current. In addition, an avalanche photo diode is a light receiving element having a light absorbing layer and an avalanche multiplication layer can realize a high-sensitivity element in comparison with a photo diode, because the avalanche photo diode uses the multiplication of carriers by avalanche multiplication. A photo diode array has a plurality of photo diodes or avalanche photo diodes which are one-dimensionally or two-dimensionally arrayed in the plane, and are used as large-area optical sensors or the like.
As an avalanche photo diode, one has been proposed in which a concave portion is formed on a multiplication layer to provide a p-type diffusion region, causing avalanche brake down to occur at the center of the p-type diffusion region (for example, refer to Japanese Patent Laid-Open No. 62-033482). Thereby, edge breakdown caused at the end portion of the p-type diffusion region due to electric field concentration can be prevented. However, to form the concave portion, the process becomes complicated, and the variation of the element characteristics may be occurred. In addition, another diode has been proposed in which an electrode and a multiplication layer are made not to be neighbor, and an electric field relaxing layer is provided to inhibit edge breakdown (for example, refer to Japanese Patent Laid-Open No. 2010-135360).
A photo diode where comb-shaped shot key electrode is provided on a light-absorbing layer has been proposed (for example, refer to Japanese Patent Laid-Open No. 2000-101130). However, since light is received at a narrow depletion region formed in the vicinity of the electrode, and cannot be received outside of the depletion region and a region shaded by the electrode, the aperture ratio is small.
A photo diode wherein the p-type region is not selectively formed, but the epitaxial grown p-type layer is made to be mesa structure by etching has been proposed (for example, refer to Japanese Patent Laid-Open No. 2001-119004). However, the p-n joint interface is exposed on the side of the mesa, and the reliability has concerns. Further, since the electrodes are wired as a matrix, the chip area is enlarged.
A photo diode array providing a light shielding film to prevent the deterioration of characteristics due to stray light incident between arrays has been proposed (for example, refer to Japanese Patent Laid-Open No. 2002-100796). However, since currents are injected from the p-type electrodes placed on the end of the light-receiving region, electric fields are not evenly applied at a position located away from the electrodes due to a voltage drop, and the band may be deteriorated. Further, since respective elements are not electrically separated, electrical cross talk may be generated.
In a photo diode array of surface incident type wherein respective arrays are not electrically separated (for example, refer to Japanese Patent Laid-Open No. 2009-38157), electrical cross talk may occur. Further, slow responding component occurs by stray light incident between arrays, and there are concerns such as the deterioration of the bands or the lowering of S/N ratio. To prevent this, it has been proposed to prevent the signal light from entering other areas than the light receiving region by means of light shielding metal (for example, refer to Japanese Patent Laid-Open Nos. 2002-100796, 63-211686, and 3-276769), or to collect light into apertures by light shielding metal (for example, refer to Japanese Patent Laid-Open No. 2007-281144).
In the back face incident type photo diode array, since there is no electrode on the light incident surface, a high aperture ratio in comparison with the surface incident type can be obtained (for example, refer to Japanese Patent Laid-Open No. 2007-281266). However, a separating channel is formed for element separation, the area of the light receiving region decreases by the amount of the separating channel, lowering the aperture ratio. To prevent the lowering of the aperture ratio, the width of the separating channel needs to be narrower. However, it is difficult to reduce the width of the separating channel to a certain width or smaller and to put a coating of surface protecting film up to the bottom of the separating channel as the wide is narrower because of side etch, and as the width is narrow, the reliability is concerned. In addition, since the p-n joint interface exists on the sides of the separating channel formed by etching, there is a concern with respect to reliability.
On the contrary, the provision of a light reflecting layer on the slope of the separating channel having a V-shaped cross section has been proposed (for example, refer to Japanese Patent Laid-Open No. 62-36858). The light incident to the separating channel is reflected by the light reflecting layer, and guided to the light-absorbing layer. Therefore, since light incident to the separating channel can also contribute to the photocurrent, a high aperture ratio can be obtained.
Prior Art Document
Patent Document